754 GASTROPODA. 



cases very minute, and hidden in the mantle (as, for example, in the 

 Slugs). From the very general occurrence of a shell which is " uni- 

 valve," or composed of a single piece, the Gastropods are commonly 

 spoken of as the " Univalve Molluscs." In its chemical composi- 

 tion the shell is composed of carbonate of lime (sometimes in the 

 condition of calcite, sometimes in that of aragonite). Its inner 

 layer is often nacreous, and it grows by additions made to its free 

 margin by the muscular edge (" collar ") of the mantle, in which 



Fig. 639. — Amfiullaria canaliculata. 0, Operculum ; s, Respiratory siphon. 



numerous pigment-glands are contained. Primitively the shell is 

 covered with a horny cuticular layer (" epidermis "), but this often 

 disappears with age. In many cases, the mantle becomes reflected 

 over the shell, the outer surface of which may thus become covered 

 with a layer of enamel (as in the Cowries). 



The shell of the Gastropods is to be regarded as essentially a 

 cone, the apex of which is more or less oblique. In the simplest 

 form of the shell, the conical shape is retained without any altera- 

 tion, as is seen in the common Limpet {Patella). In the great 

 majority of cases, however, the cone is considerably elongated, so 

 as to form a tube, which is usually coiled up into a spiral. The 

 " spiral univalve " (fig. 640) may, in fact, be looked upon as the 

 typical form of the shell in the Gastropoda. In some cases the 

 coils of the shell — termed technically the "whorls" — are hardly in 

 contact with one another (as in Vermetus). More commonly the 

 whorls are in contact, and are so amalgamated that the inner side 

 of each convolution is formed by the pre-existing whorl. In some 

 cases the whorls of the shell are coiled round a central axis in the 

 same plane, when the shell is said to be " discoidal" (as in the com- 

 mon fresh-water shell Planorbis). In most cases, however, the 

 whorls are wound round an axis in an oblique manner, a true 

 spiral being formed, and the shell becoming " turreted," " tro- 



